In recent years, substation facilities have had to be strengthened as the power facilities have been installed at remote places or as the supply of power to cities has increased. For these necessities, there have already been spread and serviced gas insulated devices such as so-called gas-insulated breakers or transformers in which a substation device such as a disconnector or breaker is housed in a closed container by using SF.sub.6 gas, which is excellent in insulation and arc extinguishing ability to improve the environmental adaptation and to decrease the installation volume per kV A. This gas-insulated device is advantageous in its compactness and in its elimination of the exposed charging unit of a grounded tank, but is defective in difficulty of diagnosis for the maintenance attributed to high performance, the increase in the time period for the maintaining and repairing works, and the marked lowering of the insulation reliability when the inside becomes abnormal. Similar problems are caused in transformers using oil insulation or in cables using solid insulation by the increase in the size of the device or in the transmission distance.
In order to improve the insulation reliability of the insulated device in its entirety, therefore, a variety of efforts have been made conventionally for appropriate design/manufacture of the device. For one improvement in the capacity of the power supply, confirmation and monitoring of the reliability of the entire device are required and various studies and investigations have been made.
One cause of the lowering of the insulation reliability is the non-uniform electric field. Especially, SF.sub.6 gas, used in the gas insulated device, exhibits especially excellent insulating characteristics in a non-uniform electric field, but the insulating characteristics drop extremely under the non-uniform electric field. The factors disturbing the field distribution in a gas insulating device can be defects such as flaws in the surface of high-voltage conductors or metal foreign matter having entered the inside during assembly or transportation. Other conceivable factors may be an imperfect contact of high-voltage conductors due to assembly mistakes or defects such as voids in the insulation spacer. If a non-uniform electric field is established in the gas insulating device by those defects, partial discharge may be caused during the operation, leading to a serious situation such as the breakdown of the entire circuit. This makes it necessary to detect partial discharge reliably before the entire circuit breakdown thereby to prevent such breakdown in advance. From this background, there have been examined methods for detecting partial discharge which may occur not only in a gas insulating device but also in any insulated device.
Of these, there is a method of detecting electromagnetic waves which are generated as a result of partial discharge, disclosed in Japanese Patent Laid-Open No. 107174/1989. By this method, there is provided a diagnosis system in which input signals containing many radio frequency band components, received by an antenna provided in an insulated device, are expanded in intensity for every frequency by a spectral analyzer, so that only the presence/absence of partial discharge is detected based on the distribution pattern and level. Another diagnosis system is disclosed in Japanese Patent Laid-Open No. 260868/1995, in which the presence/absence of partial discharge and the type of defect are judged from the distribution and intensity of such a spectrum such that period parameters such as the voltage phase angle are given to the intensity of a fixed frequency in the intensity spectrum expanded by the spectral analyzer.